1. Field of the Invention
Embodiments of the invention relate to power supply device control circuits and, in particular, power supply device control circuits that include overcurrent protection.
2. Description of the Background
An alternate current (AC)/direct current (DC) converter of a switching power supply is a circuit which converts AC voltage into DC voltage, and as a circuit system, a flyback is widely adopted.
The flyback AC/DC converter, including a flyback converter including a transformer and a switching transistor (for example, an MOSFET: a metal oxide semiconductor field effect transistor), uses the flyback converter to obtain desired DC voltage from voltage wherein AC voltage is rectified by a diode bridge.
Additionally, a control integrated circuit (IC) for carrying out a stable supply of output voltage also against a factor, such as fluctuations in AC input voltage or load, or a change in temperature, is included in this kind of AC/DC converter.
The control IC, being disposed on the primary side of a transformer in the flyback converter, feedback controls the switching of the MOSFET based on information of output voltage on the secondary side of the transformer, thereby maintaining the output voltage constant.
Meanwhile, when a load connected on the output side of the AC/DC converter becomes larger or short-circuits, there is fear that current flowing through the MOSFET or transformer in the flyback converter becomes larger, thus leading to breakage of an element.
Because of this, the control IC includes an overcurrent protection (OCP) function which protects the circuit in such a way that output current equal to or more than a specified value does not flow by limiting current flowing through the MOSFET.
As a heretofore known technology of the OCP function, there is proposed a technology wherein a peak current is made constant by level shifting the triangular wave of an oscillator and taking it as the reference voltage of the OCP function, and thereby making the reference voltage high at low input voltage and making the reference voltage low at high input voltage. See, for example, U.S. Patent Application Publication No. 2008/0291700. Also, there is proposed a technology wherein AC input voltage is detected, and the reference voltage of the OCP function is changed in accordance with the detected AC input voltage. See. for example, Japanese patent application publication no. JP-A-2002-153047.
The OCP function limits the peak current of a switching element (for example, MOSFET, hereafter referred to simply also as MOSFET on behalf of the switching element) disposed on the primary side of the transformer. Specifically, the control IC compares voltage proportional to current flowing through the MOSFET and the reference voltage, recognizes that overcurrent is generated when it detects a voltage exceeding the reference voltage, and carries out the control of stopping the switching of the MOSFET.
In this case, delay occurs until the time when the control IC actually stops the switching of the MOSFET after it has detected the overcurrent. Therefore, even when the overcurrent is detected, the switching does not stop instantaneously, and the overcurrent flows for a delay time.
Meanwhile, a change occurs in the slope of the rise of drain current flowing through the MOSFET due to a change in the AC input voltage, and the slope of the drain current is gentle when the AC input voltage is low, while the slope of the drain current is steep when the AC input voltage is high. This is due to the inductance of the transformer through which the drain current flows.
Herein, for example, when the reference voltage of the OCP function is at a constant level, a difference occurs between the maximum value (a peak current) of drain current flowing in a delay time zone in which the OCP functions when the slope of the drain current is gentle and the maximum value of drain current flowing in a delay time zone in which the OCR functions when the slope of the drain current is steep.
In this way, as a difference occurs in the drain current flowing in the delay time zone in which the OCP function works due to a change in the AC input voltage, the heretofore known technologies heretofore described (see, for example, U.S. Patent Application Publication No. 2008/0291700 and Japanese patent application publication no. JP-A-2002-153047) adopt a configuration wherein the reference voltage is also adaptively changed based on the change in the AC input voltage, thus making the peak current constant.
However, the slope of the rise of the drain current of the MOSFET also changes due to a change in the inductance value of the primary winding of the transformer in the flyback converter. Because of this, the peak current cannot maintain a constant value depending on a transformer used.
With the heretofore known technologies heretofore described, a shift of the peak current due to the change in the inductance value on the primary side of the transformer cannot be coped with, and there is a problem that it is difficult to realize an OCP function which is always stable.